1. Field of the Invention
This invention relates to an optical device for an optical transmitting device, an optical receiving device, an optical transmitting/receiving device or other optical parts for constructing same and an assembly of the devices for optical communication. This invention, in particular, aims at reducing the ORL (Optical Reflection Loss).
This application claims the priority of Japanese Patent Application No.11-196468 (196468/1999) filed Jul. 9, 1999 which is incorporated herein by reference.
2. Description of Related Art
Practical development of optical communication accelerates miniaturization and cost-reduction of optical transmitting devices, optical receiving devices or so. Recent endeavors are made for investigating very tiny optical devices called PLC (planar lightwave circuit) type which makes use of passive alignment. For example, the followings suggested PLC devices.
{circumflex over (1)} T. Nishikawa, Y. Inaba, G. Tomon, T. Uno, Y. Matsui, xe2x80x9cSurface Mounting LD Module on a Silicon Substratexe2x80x9d, 1997 IEIC C-3-63, p248(1997).
{circumflex over (2)} Jun-ichi Sasaki, Masataka Itoh, Hiroyuki Yamazaki, Masayuki Yamaguchi,xe2x80x9cSi bench for highly efficient optical coupling using passively-aligned spot-size converter integrated laser diodexe2x80x9d, 1997 IEIC C-3-65, p250(1997).
{circumflex over (3)} A. Hirai, R. Kaku, T. Maezawa, K. Takayama, T. Harada, xe2x80x9cSilicon V-Groove Substrate for Optical Modulesxe2x80x9d, 1997 IEIC C-3-66, p251(1997).
FIG. 1 and FIG. 2 show prior PLC type optical receiving modules (PD module). FIG. 1 is a plan view of the PLC type PD module and FIG. 2 is a sectional of the same module. An optical receiving module (PD module) 1 has an Si bench 2 including a lower step 4 and a higher step 3. The higher step 3 sustains an end of a fiber 9 and the lower step 4 holds a PD 5. The PD 5 is a waveguide type PD which has an light sensing waveguide 12. The light going into the PD from the side is sensed by the waveguide 12. The Si-bench 2 has a smaller V-groove 7 and a bigger V-groove 6 made by anisotropic etching on the upper step 3. A ferrule 8 and the fiber 9 are supported in the V-grooves 6 and 7. The ferrule 8 encloses an end of the fiber 9. The ferrule 8 can be attached to or detached from an external optical device (not shown in FIGS. 1 and 2). The end surface of the fiber 9 is orthogonal to the central optical axis. Outgoing light 11 from an end 10 passes a narrow gap and reaches the light sensing waveguide 12 of the PD 5. The fiber is also fixed to the same Si-bench 2. Mounting both the fiber and the PD on the same Si-bench enables the PD module to reduce its size. There is no joint requiring alignment. No alignment (passive alignment) alleviates the fabrication time and the cost. The omission of a lens reduces the cost also. Then, the PLC type PD module of FIG. 1 and FIG. 2 would be a cheap, miniaturized PD module.
The prior art of FIG. 1 and FIG. 2 disposes optical devices (PD 5, ferrule 8 and optical fiber 9) on t he Si-bench 2 for joining the fiber directly to the light sensing device (photodiode; PD 5) without lens. The butting joint between the fiber and the PD allows the PD module to decrease parts and reduce the size, which would lead to a low-cost PD module. Here the optical fiber 9 is shown as a light introducing part by way of example. A light waveguide can be employed instead of the fiber 9. The waveguide type PD which allows the incidence light to enter the side as an example. The side incidence waveguide type can also be replaced by a top incidence type PD or a bottom incidence type PD in accordance with the design of the optical system.
The V-grooves 6 and 7 are formed by anisotropic etching based on photolithography on an Si wafer. The positioning marks are formed also by photolithography on the Si wafer for predetermining the spot of a PD on a bench. The V-grooves and the positioning mark enable the module to place the fiber and the PD at exactly predetermined positions. The rigorous positioning by the grooves and the marks without positive alignment is called xe2x80x9cpassive alignmentxe2x80x9d. The passive alignment allows the PLC module denoted by FIG. 1 or FIG. 2 to reduce the assembling cost. The PLC module has advantages of low part cost and low assembling cost.
The end of the fiber is orthogonal to the light axis. The orthogonality is considered to be indispensable to the passive alignment. If the end surface were to be oblique to the light axis, the beam emanating from the fiber would bend sideward and would require a time-consuming positive alignment for coupling with the PD. It is a common sense that the passive alignment should inherently request the orthogonal end of the fiber.
As a matter of fact, the reflection at the fiber end causes a problem in the PLC prior art of FIG. 1 and FIG. 2. The end 10 of the fiber is cut in a plane vertical to the light propagating direction (axial direction). Another end of the fiber faces an LD (laser diode) as a light source (not shown in the figures). The vertical end reflects the laser light backward. The reflected beam propagates in the fiber in the reverse direction to the LD and induces instability of the LD oscillation. The LD makes use of mirror surfaces of both ends for reciprocating light as a resonator. If the light reflected at the fiber end returned to the LD, the LD would have two resonators. The existence of two resonators changes the oscillation wavelength or the frequency and the power. The instability would incur inconveniences. The returning light should be fully suppressed for maintaining the stability of the laser oscillation-wavelength and power. The light receiving surface of the PD which is coated with an antireflection film does not reflect the LD light. But the fiber end which is not coated with the antireflection film would cause the serious problem by reflecting the laser light.
The light which is reflected by the fiber end and is returned to the LD is called xe2x80x9creflection returning lightxe2x80x9d here. The light emanating from the LD is called xe2x80x9cinput lightxe2x80x9d. The rate of the reflection returning light to the input light is called ORL (Optical Reflection Loss).
ORL=10 log(Pr/Pin)(dB).xe2x80x83xe2x80x83(1)
Here, log means logarithm, Pr is the light power which is reflected at the fiber end and is returned to the LD and Pin is the light power which is produced by the LD and is progressing to the fiber end. ORL is defined in a unit of dB. Since Pr is always smaller than Pin(Pr less than Pin), ORL is negative. ORL is a measure of the influence of the returning light to the LD. Smaller ORL is better for the PD module. Too big ORL is a drawback of the prior art of FIG. 1 and FIG. 2. The ORL of the PLC module is now calculated.
The power reflection rate Ref at an interface from a medium of a refractive index n1 to another medium of a refractive index na is denoted by
Ref={(n1xe2x88x92na)/(n1+na)}2.xe2x80x83xe2x80x83(2)
In the case of the prior PD module shown by FIG. 1 and FIG. 2, light goes out from a quartz fiber of a refractive index n1=1.46 to air of a refractive index na=1.00. The returning light is ORL=xe2x88x9214.6 dB which is a large value. Namely, the reflected light is strong. The large difference of refractive index between the fiber and air leads to such a big ORL.
How small ORL is required for practical PD modules? The requirements and characteristics depend upon the kinds of optical communication systems. The allowable maximum ORL is contingent upon the systems. More sophisticated system requires a smaller ORL. An optical receiving device requires a small ORL of less than xe2x88x9227 dB. The fabrication margin is about xe2x88x923 dB. Then, less than xe2x88x9230 dB is required for the ORL in practice. This is why the laser is perturbed even by small reflection returning light.
Furthermore, transmission of multichannel analog signals, e.g., optical CATV, requires a very small ORL of less than xe2x88x9240 dB. This is a very rigorous requirement itself It is yet not sufficient that the ORL is less than xe2x88x9240 dB at a certain temperature. The scope of temperature in which optical modules should operate regularly ranges from xe2x88x9240xc2x0 C. to +85xc2x0 C. The ORL should be less than xe2x88x9240 dB in whole the temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C. This is a quite rigorous condition.
The suggested PLC of FIG. 1 and FIG. 2 cannot satisfy the requirement, since ORL=xe2x88x9214.6 dB. The PD module of FIG. 1 and FIG. 2 has poor utility due to the large reflection returning light. The application of the prior PD module is narrowly restricted within transmission of digital signals with a slow rate in few channels. Trials have been done for reducing ORL far lower than the prior art.
FIG. 3 and FIG. 4 show a contrivance filling the space between the fiber 5 and the PD 5 with a transparent resin 14 having a refractive index nearly equal to the fiber for reducing the reflection loss at the fiber end. The occupying resin which is called a xe2x80x9cpotting resinxe2x80x9d alleviates the reflection loss at the fiber end. For example, the following documents suggested the prior art of FIG. 3 and FIG. 4.
{circumflex over (4)} T. Ishii, S. Eguchi, K. Yoshida, T. Kato, K. Fukuda, T. Ishikawa, xe2x80x9cPigtail Type Optical Module by Transfer Moldingxe2x80x9d, 1997 IEIC C-3-62, p247 (1997).
{circumflex over (5)} K. Yoshida, T. Kato, T. Hirataka, F. Yuuki, K. Tatsuno, T. Miura, xe2x80x9cOptical Coupling Properties of LD module using resin moldingxe2x80x9d, 1997 IEIC C-3-68, p253 (1997).
{circumflex over (6)} Kazuyoshi Hasegawa, Masayuki Kubota, Japanese Patent No.2792722, xe2x80x9cSemiconductor Light Emitting Devicexe2x80x9d.
Eq.(2) teaches us that the difference (n1xe2x88x92na) of the refractive index is the origin of reflection. Reduction of the refractive index difference (n1xe2x88x92na) decreases the reflection at the interface. In general, silicone-group resins or acrylate-group resins are sometimes used for potting (coating) electronics parts to protect the object parts from humidity or oxidization. In the prior art of FIG. 3 and FIG. 4, the purpose of filling the potting resin 14 between the PD and the fiber is not to protect the devices but to reduce the ORL. The conditions imposed upon the resin are transparency for signal light and refractive index similar to the fiber. The silicone-group resins or acrylate-group resins are transparent not only to visible light but also to 1.3 xcexcm light and 1.55 xcexcm light suitable for optical communication.
These transparent resins have refractive indexes akin to the optical fiber (1.46). For example, the silicone-group resins have a refractive index about n=1.4 at room temperature. The acrylate-group resins have a refractive index about n=1.5 at room temperature. At room temperature, by chance, both the silicone-group resins and the acrylate-group resins can satisfy the aforementioned requirement of the ORL less than xe2x88x9230 dB (ORLxe2x89xa6xe2x88x9230 dB). The PD module of FIG. 3 and FIG. 4 has a fiber end vertical to the light axis, because the module is made by passive alignment.
Refractive index of any resin has temperature dependence. Any existent resin material cannot satisfy the condition (ORLxe2x89xa6xe2x88x9230 dB) throughout the wide temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C. FIG. 5 is a graph showing the ORL of an acrylate-group resin (⋄) and a silicone-group resin (xe2x97xaf) as a function of temperature. The abscissa is temperature (xc2x0C.). The ordinate is ORL (dB). The silicone-group resins or the acrylate-group resins include some different species with different refractive indexes. FIG. 5 shows only an example belonging to the groups. In general, a rise of temperature reduces the ORL in the acrylate-group resin. A decrease of temperature reduces the ORL in the silicone-group resin. Both the resin groups cannot easily satisfy the condition (ORLxe2x89xa6xe2x88x9230 dB) throughout the temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C. Both resin groups cannot satisfy the more rigorous condition ORLxe2x89xa6xe2x88x9240 dB required for analog signals.
The change of the ORL depending on temperature is caused by the change of the refractive index as a function of temperature. In FIG. 5, the temperature dependence of the ORLs is contradictory between the silicone-group resin and the acrylate resin. The inverse change of the ORLs does not derive from the difference of the temperature tendency of the refractive indexes. On the contrary, the refractive indexes decrease with a rise of temperature both for the acrylate resin and the silicone resin. The silicone resin continually changes the refractive index from 1.48 to 1.37 in the temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C. The acrylate resin continuously changes the refractive index from 1.56 to 1.49 in the temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C. The refractive index of the fiber is 1.46. The silicone resin separates the refractive index farther from 1.46 (fiber) in a rise of temperature, which increases the ORL for the silicone resin. The acrylate resin pushes down the refractive index closer to 1.46 (fiber) in a rise of temperature, which decreases the ORL for the acrylate resin. There are resins which have a refractive index nearly equal to that of the quartz fiber. However, the change of temperature varies the ORL. The above explanation relates to the difficulty of coupling the fiber with the PD. The difficulty accompanies also the coupling of the light waveguide with the PD. In the case of the Si light waveguide, the reflection returning light disturbs the laser oscillation which causes malfunction of the optical devices.
The prior art PD module of FIG. 1 and FIG. 2 having an air gap has only an improvement of FIG. 3 and FIG. 4 of filling the air gap with the transparent resin as a remedy for protecting the LD from the reflection returning light. The transparent resin can reduce the reflection. But the temperature variation disturbs the function of the resin through the change of refractive index. Even if the resin-potting PD module partially could satisfy the ORL less than xe2x88x9230 dB in the full temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C., the resin-filled module cannot fulfill the severe, future requirement of the ORL less than xe2x88x9240 dB.
This invention proposes a coupling between a fiber (or a waveguide) and an optical device produced by cutting an end of the fiber (or the waveguide) slantingly and filling a gap with a transparent (potting) resin of a refractive index akin to the fiber (or the waveguide). The end of the fiber is not cut into an orthogonal surface to the axis but cut into a slanting surface. The fiber or the waveguide allows light to pass in a single way in a definite direction. The light axis can be defined by the fiber or the waveguide. The slanting angle xcex1 is defined as a deviation angle from the orthogonal plane. The important matter is the slanting end surface and the potting resin for the present invention.
This invention features two means: slanting end and potting resin, for suppressing the reflection returning light. The cutting angle of the fiber end or the waveguide end should be about 2 degrees to 10 degrees. Preferable angle is 2 degrees to 8 degrees. The slanting cut of the end hinders the reflected light from returning back in the same fiber to the LD. The reflected light is extinguished at the end of the fiber or the waveguide. Furthermore, the encapsulation of the gap by the transparent resin of a refractive index akin to the fiber or waveguide alleviates the reflection itself The resin encapsulation is known in the PLC module as shown in FIG. 3 and FIG. 4. The slanting end is also well known in hermetic seal modules enclosed with metal package. But the combination of the resin encapsulation and the slanting end is quite novel in the PLC type modules.
This invention can be applied to various optical devices. Before many examples are explained, a typical example is first described for facilitating to understand the feature of the present invention. FIG. 6 and FIG. 7 denote an example (fiber+waveguide type PD) of the present invention. Like prior art modules of FIG. 1 to FIG. 4, a Si bench 2 has an upper step 3 and a lower step 4. The upper step 3 has a larger V-groove 6 and a smaller V-groove 7 made by anisotropic etching of the single crystal silicon bench 2. A ferrule 8 and a fiber 9 are fixed in the V-grooves 6 and 7. The ferrule 8 is a cylinder holding an end of the fiber 9 coaxially for allowing external parts to attach to or detach from the fiber. An inner end 16 of the fiber 9 is cut slantingly. The slanting angle (xcex1) is, for example, 4 degrees, 6 degrees, 8 degrees or so. A waveguide type PD 5 is mounted at a definite spot upon the lower step 4. Positioning marks denote the predetermined position of the PD 5. The end 16 of the slanting cut fiber is covered by a transparent potting resin 14 having a refractive index similar to the fiber. Covering solely the end is still effective. Preferably, the gap between the fiber end 16 and the receiving surface of the PD 5 should be fully covered with the potting resin 14.
This invention has the gist of cutting slantingly the end of the fiber in addition to the coating with the transparent resin. The slanting end cut has neither been done nor suggested till now in the field of the PLC technology. The slanting end cut is quite new in PLC modules. The slanting end has been, however, a commonplace in a neighboring field of technology of conventional metal can hermetic shielded devices. The conventional metal canned, three-dimensional devices make the best use of the slanting cut fiber end for suppressing the reflected light from returning to the LD. FIG. 8 shows a prior PD module hermetically shielded in a metal package.
The prior PD module has a metal round stem 20. The metal stem 20 has an insulating submount 21 at the center on the upper surface. A top-incidence type PD 22 is mounted upon the submount 21. Wirebonding connects a lead pin 31 to the submount 21 and a lead pin 33 to a PD top electrode with wires. A cylindrical cap 23 with a lens 24 is welded on the upper surface of the stem 20. A cylindrical sleeve 25 is fitted on the stem 20 for covering the cap 23. A ferrule 26 holding an end of an optical fiber 27 is inserted into an axial hole 28 of the sleeve 25. The lower end 30 of the fiber 27 and the ferrule 28 is polished in a slanting angle. A bend limiter 29 is capped on the sleeve 25 for preventing the fiber from bending excessively. Since the end of the fiber is cut slantingly, the outgoing beam is refracted to the left (toward the lower side of the slanting end). The outgoing beam does not fall along the axial direction but falls slantingly to the left. Then, the sleeve 25 is aligned by moving the sleeve 25 on the stem 20 two-dimensionally, observing the power sensed by the PD 22, searching the position and the rotation angle of the sleeve for obtaining the maximum power and welding the sleeve at the position. This is the horizontal alignment. Further, the ferrule 26 is positioned at an optimum depth by moving the ferrule 26 in the axial hole, measuring the power of the PD, determining the depth of the ferrule for obtaining the maximum power and welding the ferrule 26 to the sleeve 25. This is the vertical alignment. The alignment is indispensable for the metal packaged hermetic sealed PD module having the oblique fiber end. The alignment is difficult and time-consuming work, which raises the cost of optical devices.
The above device is sometimes called a coaxial type PD module, since it includes concentric parts (cap, lens, sleeve, ferrule, bend limiter) to the central light axis. The module has three dimensional structure in which the axial line meets at right angels to the PD chip. The example has a fiber end of a slanting angle of 8 degrees. The slanting end is a contrivance for prohibiting the light reflected at the end from going back through the fiber to the LD. The outgoing beam is refracted to the left and deviates to the left from the axial line. The lens and the PD do not exist on an extension of the fiber axial line but lie at spots deviating to the left from the extension. In fabricating such a three-dimensional device, the cap and the sleeve are two-dimensionally aligned and welded on the stem after bonding the PD on the stem. Such alignment allows the fiber end to be cut slantingly. Without alignment, the slant cut fiber end would not prohibited to the contrary. The alignment operation determines the optimum positions of the cap 23 (lens 24) and the sleeve 25 (fiber 27) with respect to the stem 20 for the PD to receive the maximum power from the fiber. The alignment enables the PD module to obtain high sensitivity and low ORL at the same time. The operation of monitoring the PD, displacing three-dimensionally the cap, the ferrule and the sleeve and seeking the optimum positions is called active alignment. Such time-consuming active alignment itself allows the slanting end cut of the fiber. If the parts were not aligned actively, the slanting cut end could not join the PD in a good condition. The active alignment is an antonym of the passive alignment of the PLC devices which would forbid the slanting cut of the fiber.
Such a PD module which requires time-consuming active alignment raises the production cost. The expensive PD module would be a hindrance for building inexpensive optical communication systems. Cheap PLC type devices shown in FIG. 1 to FIG. 4 are still desirable. The PLC type PD module of FIG. 1 to FIG. 4 dispenses with the time-consuming active alignment. Since the alignment is omitted, the positioning of the PLC is called xe2x80x9cpassive alignmentxe2x80x9d. xe2x80x9cPassivexe2x80x9d alignment simply means xe2x80x9cnoxe2x80x9d alignment. In the PLC module of FIG. 1 to FIG. 4 does not divert the PD from an extension of the fiber axis. Thus, the skilled persons have considered that there would be no room for including the step of cutting the fiber end obliquely in the production of the PLCs. In the PLC module, in the first place, the PD is made at the spot coinciding with an extension of the fiber central axial line. They consider that if the fiber end were cut slantingly, the light emanating from the fiber would bend sideways and would never go into the PD on the PLC module. Such a sturdy belief has severely forbidden the PLC to cut the fiber end obliquely.
The Inventors think otherwise. The Inventors consider that the oblique cut end will be still effective for the PLC type devices which refuse the active alignment. The slanting end cut will be effective even for the PLC devices as long as the transparent potting resin is used for covering the fiber end. Cooperation of the slanting end cut and the potting resin enables this invention to accomplish an unexpected effect.
The deviation angle by the refraction at the fiber end is explained by referring to FIG. 9 for clarifying the concept of the present invention. The refractive index of the fiber core is denoted by n1. The refractive index of the outer medium (potting resin) is denoted by na. The central light axis determined by the fiber is designated by KMN. M is a middle point of the slanting end surface 16 of the fiber. The outlet surface 16 is not orthogonal but slanting by xcex1 to an orthogonal plane MC. Namely, ∠DMC=xcex1. NF is a normal standing on the slanting plane 16 at M. The normal MF inclines by xcex1 to the light axis MN. The fiber propagating beam KM is refracted at M into an outgoing beam MG. A beam reflected at M is denoted by MR. The reflected beam MR is important. The matter is whether the reflected beam MR returns to the LD or not. The refraction is complex but the reflection is simple. The inclination angle of the reflected beam MR to the light axis MK is simply 2xcex1. Namely, ∠KMR=2xcex1.
The refraction is more complicated than the reflection. The inclination angle of MG to the normal MF is denoted by xcex2. The beam MG deviated from the axial line MN at xcex8. xcex2=xcex8+xcex1. To the refraction KMG, Snell""s law gives a relation between xcex1 and xcex2,
n1 sin xcex1=na sin xcex2.xe2x80x83xe2x80x83(3)
xcex2 and xcex8 are written as,
xcex2=sinxe2x88x921(n1 sin xcex1/na),xe2x80x83xe2x80x83(4)
xcex8=sinxe2x88x921(n1 sin xcex1/na)xe2x88x92xcex1.xe2x80x83xe2x80x83(5)
xcex8 is a deviation angle of the outgoing beam MG from the light axis MN. If na were equal to n1(na=n1), the deviation angle would be zero. The closer the medium refractive index na approaches to the fiber refractive index n1, the smaller the deviation angle xcex8 reduces. On the contrary, the deviation angle xcex8 increases as the medium refractive index na separates farther from the fiber refractive index n1.
The reflection angle is simply ∠KMR=2xcex1. Whether the reflection beam can become a propagation beam in the fiber depends upon a relation between the core refractive index and the cladding refractive index. The core refractive index is n1. The cladding refractive index is n2. Of course, n1 greater than n2. A full reflection angle xcexa8 is defined as a critical angle for the fiber. An inclination angle is defined as an angle between the beam line and the central axial line. The beams having an inclination angle of less than xcexa8 can become a propagation beam in the fiber. The beams having an inclination angle of more than xcexa8 escapes from the fiber and does not become a propagation beam. xcexa8 is given by the full reflection condition at an interface between the core and the cladding. When the core beam is slanting by xcexa8 to the normal standing on the interface, the refracted cladding beam goes just in the interface. Namely, Snell""s law requires n1 sin{(xcfx80/2)xe2x88x92xcexa8}=n2 sin(xcfx80/2). sin{(xcfx80/2)xe2x88x92xcexa8}=cos xcexa8. Then,
cos xcexa8=n2/n1.xe2x80x83xe2x80x83(6)
Since the refractive indices of the core and the cladding are very close together, xcexa8 is a small angle. A single-mode fiber has a quite small xcexa8. In FIG. 9, the slanting angle of a reflected beam is 2xcex1. If 2xcex1 less than xcexa8, the reflected light can become propagating light, because it is fully reflected at the interface. This is called a propagation mode. But if 2xcex1 greater than xcexa8, the reflected light cannot become propagating light, because the light leaks into the cladding and dies away. This is called a dissipation mode. The explanation of the selection based on geometric optics is simple. Strictly speaking, the beam has variation of power distribution. The dynamics would be rigorously treated by wave optics. But the selection whether the reflected light would be a propagating mode or dissipating mode can be judged only by comparing 2xcex1 with xcexa8. The prior art of FIG. 1 to FIG. 4 having vertical ends chose xcex1=0. Thus, all the reflected light becomes the propagation mode. This invention gives a small definite value to xcexa8 for annihilating the reflection light of 2xcex1 greater than xcexa8. This invention preventing the reflected light of 2xcex1 greater than xcexa8 from returning to the LD. The allowable lower limit of xcex1 is xcexa8/2=(1/2)cosxe2x88x921(n2/n1).
The allowable lower limit is, for example, two degrees. In the case of a single mode fiber, xcexa8/2 is less than 2 degrees. Thus, a slanting angle xcex1 more than 2 degrees can completely exclude the reflected light. Since the reflection is based upon the simple law, the problem of reflection is apt to be neglected by being occulted by refraction problems. The explanation hitherto relates to the concrete condition whether the reflected light can be returning light or not. The reflected returning light is the first important matter of the present invention.
Another problem is the refraction at the fiber end. For example, when light is emitted from an end of an optical fiber (n1=1.46) into air (na=1.00), the refracted light will incline at 1.85 degrees from the light axis in the case of the slanting angle xcex1=4xc2x0. If the distance between the fiber end and the PD is 500 xcexcm, the refracted beam would deviate sideways from the center of the PD by about 16 xcexcm. Such a large deviation hinders the beam from entering the light receiving part of the PD. The waveguide type PD of FIG. 1 to FIG. 4 has about a 2 xcexcm to 5 xcexcm width of waveguide. The tolerance of the scope within a 1 dB sensitivity fall is a few micrometers (xcexcm) for the waveguide type PD. The PD does not sense the 16 xcexcm deviating beam at all. This calculation teaches us that the passive alignment is forbidden in the case of the outer medium of air (na=1.00).
The calculation was based upon the assumption of the 500 xcexcm distance from the fiber to the PD. 500 xcexcm was still a short fiber/PD distance. However, there is a margin for reducing the distance. The PD receiving power would be increased by shortening the fiber/PD distance. A 1 dB decrease corresponds to a few micrometer deviation. Suppression of the deviation of the beam from the light axis less than 3.2 xcexcm would require an extremely short fiber/PD distance of L=100 xcexcm. Such too short a distance would raise the difficulty of assembling the device. It is undesirable that the rotation of the fiber would vary the light power entering the PD. Instead of 4 degrees (xcex1=4xc2x0), if the slanting angle is 8 degrees (xcex1=8xc2x0), the deviation angle would rise to xcex8=3.72xc2x0. Even if the fiber/PD distance were extremely shortened to L=100 xcexcm, the deviation would be 6.5 xcexcm at the surface of the PD. 6.5 xcexcm is larger than the tolerance. Little light goes into the PD. Such a large deviation would deny the probability of passive alignment. If the PD should be positioned by active alignment in PLC devices, the active alignment would raise the difficulty of industrial production of the PLC devices. The above consideration seems to clarify the incompetence of a slanting fiber end for PLC devices. Perhaps no skilled person has tried to cut a fiber end slantingly for PLC devices for the reason. But the incompetence does not derive from the PLC itself but from the outer medium.
If a transparent potting resin of refractive index n=1.40 filled the light path between the fiber and the PD, the reflection itself would decrease conspicuously from Eq.(2). A decrease of reflection would induce a decline of returning light. A further important matter is that the inclination angle 2xcex1 of the reflected light exceeds the full-reflection critical angle xcexa8 and the reflected light can not be propagating light in the fiber. The reflected light becomes the dissipation mode. Since the light is rapidly dissipated in the fiber, the light cannot return to the LD. The returning light would be decreased to be nearly zero by two reasons. One is the potting resin coating. The other is the slanting cut end. Since no reflected light returns to the LD, the LD is fully immune from the operation instability. This is an important feature. In addition to the small reflection, the deviation xcex8 of the refracted beam MG from the light axis MN is also quite small. This is another important feature. Coating of the fiber end with a potting resin of a refractive index akin to the fiber exhibits three strong points: decrease of reflection, conversion of reflection light to dissipation mode and decrease of beam deviation. The former two points decrease the returning light to zero. The last point gives the possibility of passive alignment to the slanting fiber end module. They are excellent features.
For example, when the fiber end is protected with the transparent resin of na=1.40, four degree slanting cut end (xcex1=4xc2x0) bends the refracted beam MG only slightly at xcex8=0.17xc2x0 from the light axis MN which is far smaller than 1.85xc2x0 for air (n=1.00). For instance, if the fiber-PD distance is L=500 xcexcm, the deviation of the beam spot on the PD surface is only 1.5 xcexcm. The 1.5 xcexcm deviation is smaller than the 1 dB tolerance (2-3 xcexcm). Otherwise if the fiber-PD distance is L=100 xcexcm, the spot deviation is further reduced to 0.3 xcexcm. Such a small deviation allows passive alignment for assembling the device. The potting resin enables the passive aligned PD module to introduce sufficient light from the oblique fiber end into the PD. Furthermore, the once entering light becomes propagating, effective light in the PD with little loss. For L=300 xcexcm, the spot deviation is 1.9 xcexcm which still allows the passive alignment.
The above explanation relates to the waveguide type PD which has a narrow tolerance for the spot deviation. A top incidence type PD with a top inlet or a bottom incidence type PD with a bottom entrance have wider tolerances for the beam spot deviation. However, the top incidence type or the bottom incidence type PDs require longer fiber-PD distance. The longer distance compensates the wider tolerance. The effect of the potting resin is nearly equivalent for the waveguide PD, the top incidence PD or the bottom incidence PD.
Another effect of the resin coating of the fiber end is the possibility of reduction of the slanting angle xcex1. A smaller oblique angle a can accomplish a similar ORL to the prior art having a larger slanting angle of air medium. The returning power, that is, the ORL is obtained by calculating the coefficient of the coupling of an obliquely-reflected and fiber-returning Gaussian beam to the LD. FIG. 10 shows a relation between the slanting angle and the ORL with a parameter of the resin refractive index. The abscissa is the slanting cut angle xcex1 (degree). The ordinate is the ORL (dB). The refraction index of the fiber is n1=1.46. There is a parameter which is a refractive index of the medium enclosing the fiber end. The medium refractive index na is assumed to be 1.00, 1.37, 1.40, 1.56 and 1.50. 1.00 is the air refractive index. Others are refractive indexes of resins. Four parameters do not signify to compare four different resins but to consider two aforementioned resins at two different temperatures.
The ORL is the largest for the air medium case (na=1.00) denoted by black lozenges. The second largest ORL is given by a resin having na=1.56, which is shown by black rounds. The na=1.56 resin case, even if the end is perpendicular (xcex1=0), the ORL takes a small value of xe2x88x9229 dB. The fall is caused by the enclosing resin having a refractive index akin to the fiber. The falls on the xcex1=0 line (ORL-axis) are all originated from the resin enclosure irrespective of the fiber end geometry.
An increase of the slanting angle xcex1 reduces the ORL. The reduction results from the decrease of the reflection. The reduction is common for all the media and all the refractive indexes.
The third largest ORL is given by a resin of a refractive index na=1.37, which is denoted by blank squares. The 0 slanting angel xcex1=0 gives xe2x88x9231 dB. The ORL falls as xcex1 increases.
Another refractive index na=1.40 further suppresses the ORL down to xe2x88x9234 dB at the 0 slanting angle. Blank triangles denote the ORL for na=1.40. A rise of a reduces the ORL. The lowest ORL is given by na=1.50, which is designated by blank rounds. The 0 slanting angle gives xe2x88x9237 dB of ORL. The ORL decreases in an order of the refractive indexes na closing to the fiber refractive index (n=1.46).
Among the five refractive indexes, na=1.50 is the closest to the fiber index n=1.46. It is a matter of course, the resin of na=1.50 brings about the lowest ORL for all xcex1. The prior art of FIG. 3 and FIG. 4 tried to attenuate the reflection only by the action of the transparent potting resin. The declines of ORL only correspond to the falls on the ORL-axis from the black lozenges to other symbols of dots, which take all xcex1=0xc2x0. Unlike the FIG. 3 and FIG. 4 prior art, this invention makes the best use of the obliqueness of the fiber end which produces more effective falls of ORL. This invention denies xcex1=0 and proposes a slanting cut end xcex1=2 degrees to 10 degrees. For instance, xcex1 of 4 degrees enables the module to decrease the ORL by about xe2x88x9215 dB in comparison to xcex1=0. This is a conspicuous advantage of the present invention.
FIG. 10 implies that air (n=1.00) as medium would require the slanting angle xcex1=4 degrees for reducing the reflection till ORLxe2x89xa6xe2x88x9230 dB. Air would further require xcex1=6 degrees for reducing to ORLxe2x89xa6xe2x88x9240 dB. On the contrary, the transparent potting resin enables this invention to alleviate the request for the slanting angle xcex1. The minimum slanting angle 2 degrees (xcex1=2xc2x0) allows the resins of a refractive index from na=1.37 to na=1.56 to reduce the ORL till ORLxe2x89xa6xe2x88x9230 dB in the full temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C. The 4 degree cut end (xcex1=4xc2x0) satisfies ORLxe2x89xa6xe2x88x9240 dB.
The fall of ORL results from the decline of the reflection caused by a reduction of the difference of refractive index between the fiber and the medium. The reflection rate is 3.5% for air as a medium. The reflection is reduced to 0.11% for n=1.56. The reduction of the reflection is xe2x88x9215 dB. The resin coating induces such a reduction of reflection. The ORL is reduced by the same amount (xe2x88x9215 dB) as the reflection reduction. A fall of ORL is caused by a rising xcex1 and a closing na to 1.46. The extra fall by the resin coating alleviates the request of the slanting angel xcex1 for satisfying ORLxe2x89xa6xe2x88x9240 dB. The resin decreases the returning light by reducing the reflection. The reduction of reflection increases the signal light going into the PD. In this case, the PD entering light is enhanced by 3.5%. The potting resin has another effect of raising the coupling efficiency.
Experiments were carried out by making use of fibers having slanting cut ends of xcex1=2 degrees, 4 degrees and 6 degrees for confirming the result of the calculation. Similar values of ORL to the calculation of FIG. 10 are obtained in the experiments in both the case with the resin coating and the case without resin coating (n=1.00).
The present invention succeeds in reducing the ORL and diminishing the beam deviation from the light axis by cutting the fiber end obliquely and covering the end with a transparent resin. Small beam deviation allows the module to adopt the passive alignment. The present invention has advantages of low ORL, high coupling coefficient and possible passive alignment. The advantages enable the present invention to give low cost and high performance optical devices. The beam deviation (beam inclination) is so small that little attention should be paid to the direction of the slanting cut in assembling a fiber to the device. The small beam inclination dispenses with the rotation alignment of fibers. The feature facilitates the fabrication.
This invention can be applied widely to coupling between a general optical part and a fiber/waveguide. The coupling has different kinds of optical elements. One is a linear light guide for conveying light along a central axis. The linear light guide can define a central light axis which determines the propagation path of light. The linear light guide is a fiber or a waveguide.
The counterpart is an optical positive device which has some positive role. The counterpart optical part does not necessarily have an inherent light axis. The optical part is a PD in the above examples. But the optical part is not restricted to the PD. Instead of the PD, an LED or an LD can be a counterpart to the linear light guide. Other examples of the optical parts are a lens, a prism or a mirror. Namely, the linear light guides are an optical fiber and a waveguide. The optical parts are a PD, an LED, an LD, an APD, a mirror, a prism, a mirror or so.
This invention can be applied to a waveguide made on a substrate. When the optical part is an LD or an LED, the linear light guide (fiber or waveguide) carries transmitting light. In the case, the present invention reduces ORL. The ORL should be defined in a reverse relation. The ORL should be reduced also in the case for preventing instability of the LD. For instance, the case of an LD is explained by referring to FIG. 11. FIG. 11 shows a prior coupling between an LD 34 and a fiber 35 having a vertical end. The LD34 emits signal light 37 from a stripe 36. The signal light 37 goes into a fiber 35 as propagating light 38. A part of the light is reflected at the vertical end of the fiber. A reflected beam 39 returns to an end 43 of the LD 34 and induces instability in the LD. The instability is exhibited in FIG. 12 and FIG. 13. FIG. 12 shows the relation between the laser power and the driving current. The ideal case shows a linear relation. But FIG. 12 shows kinks appearing in the current/power curve as a deviation from the linear relation. FIG. 13 is an LD power spectrum having several oscillation lines. FIG. 13 shows two groups 40 and 41 of oscillation wavelengths, which invites two wavelength oscillation.
Thus, the reflected returning light is still a problem in the LD module of FIG. 11. Prior art tries to avoid the returning light by covering the path with a resin like FIG. 3 and FIG. 4. FIG. 14 shows a prior art LD module filling the gap with a transparent resin. Since a fiber end 44 is orthogonal to the beam axis, reflected light 39 returns to an LD 34, which causes oscillation instability. High power LDs producing light of more than 1 mW would be plagued by the instability induced by large reflection returning light which increases in high speed operation of more than 1 GHz. The reflection returning light increases in proportion to the laser power itself. The oscillation instability causes more serious influence upon higher speed operation. The returning light incurs an increase of noise, deformation of signals, incapability of long distance transmission or so. The reduction of the reflected returning light is earnestly requested, in particular, for DFB lasers (distributed feedback lasers) which have been utilized for high-speed, long-distance transmission.
The present invention prevents the reflected light from returning to an LD 34 by cutting obliquely an end 44 of a fiber 35 and filling the gap between the LD 34 and the fiber end 44 with a transparent potting resin 42, as shown in FIG. 15. The optics for reducing the returning light is similar to the aforementioned example of a PD module. The oblique end 44 reflects laser light 37 sideways into a sidelong beam 39. The resin 42 decreases the reflection, as Eq.(2) shows. The oblique end may invite an anxiety of a probable fall of the coupling efficiency. It matters little as explained afterward.
A question may emerge. Why do nobody hit an idea of the present invention? This invention is only a sum of two well known contrivances: slanting end cut and resin coating. Perhaps anybody has a sturdy, stale belief of, slanting end cut=beam deviation from the axis=requisite active alignment=impossible passive alignment.
The Inventors succeeded in putting the novel idea into practice both on theory and on experiment by conquering the sterile belief.
The key point of the present invention is the transparent resin. The essence of the problem, however, is the asymmetry between refraction and reflection. Both refraction and reflection are optical laws. But asymmetry discerns between refraction and reflection. The refraction angle depends upon the refractive index of media. The reflection angle is free from the refractive index of media. The refraction obeys Snell""s law. But the reflection obeys a simple reflection law that the reflection angle is minus of the incidence angle. In the case of FIG. 9,
reflection angel is 2xcex1,xe2x80x83xe2x80x83(7)
and
refraction angle is xcex8=sinxe2x88x921(n1 sin xcex1/na)xe2x88x92xcex1.xe2x80x83xe2x80x83(8)
If the media are changed to other materials, the reflection angle is still 2xcex1. The reflection angle is always 2xcex1 irrespective of the media. If the reflection angle is larger than the full-reflection angle xcexa8(2xcex1 greater than xcexa8), the reflection light cannot be returning light. The reflection light is dissipated. This fact is true for any materials. Then, the slanting fiber end can inhibit the reflected light from returning to the LD.
On the contrary, the refraction angle xcex8 is varied by the refractive index of the media. Fortunately, the refraction angle xcex8 decreases nearly to zero, if the refractive index na is close to the refractive index of the fiber. Despite the variation of xcex1, the refractive angle is nearly equal to zero (xcex8≈0) in the case of the medium having the refractive index akin to the fiber. The nearly zero refractive angle enables the refracted beam to enter directly into the PD in the case of a PD module. The about zero refractive angle allows the incidence beam from an LD to enter into a fiber without inclination and to be a propagating beam in the fiber in the case of an LD module. The nearly zero refraction angle permits the passive alignment despite the slanting end cut.
Active alignment was indispensable for the prior art of FIG. 8 having the slanting fiber end 30. What requires the active alignment is nitrogen (or air; n=1.00) as a medium. In spite of the slanting fiber end 30, if the medium were a transparent resin having a refractive index akin to the fiber, the refraction angle would be nearly zero. The nearly zero refraction angle would be able to omit the active alignment. However, the fact proceeded otherwise. The skilled in art could not break down the sturdy belief of xe2x80x9cslanting end=alignment indispensablexe2x80x9d due to rich accumulation of technical knowledge.
A glance is taken at the advantages of the present invention. This invention solves the problem of the reflection returning light in a device including a fiber/waveguide and an optical device (PD, LD or so) by cutting the end of an optical fiber or a light waveguide and enclosing the fiber/waveguide, the optical device and a space between them with a transparent resin of a refractive index akin to the fiber/waveguide. The slanting end cutting and the transparent resin cooperate with each other to annihilate the reflection returning light perfectly without reducing the coupling coefficient. The present invention succeeds in decreasing the ORL far smaller than the prior art. Fabrication of devices requiring rigorous exclusion of the reflected returning light can make the best use of the present invention. A severer requirement will be imposed on the ORL in the future. This invention will be able to respond to the future request for the ORL. This invention is suitable for sophisticated devices treating with signals of ultrahigh frequency.
In spite of the slanting end, the present invention dispenses with the active alignment, since the transparent resin suppresses the refraction angle. The low refraction angle enables this invention to serve low cost optical devices by taking passive alignment (denial of the active alignment). This invention can be applied to making PLC devices. This invention is effective for miniaturizing the optical devices.